Variable resistance memory with lattice array using enclosing transistors

ABSTRACT

A variable resistance memory array, programming a variable resistance memory element and methods of forming the array. A variable resistance memory array is formed with a plurality of word line transistors surrounding each phase change memory element. To program a selected variable resistance memory element, all of the bitlines are grounded or biased at the same voltage. A top electrode select line that is in contact with the selected variable resistance memory element is selected. The word line having the word line transistors surrounding the selected variable resistance memory element are turned on to supply programming current to the element. Current flows from the selected top electrode select line through the variable resistance memory element into the common source/drain region of the surrounding word line transistors, across the transistors to the nearest bitline contacts. The word lines are patterned in various lattice configurations.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. application Ser. No.12/888,201, filed Sep. 22, 2010, which is a division of U.S. applicationSer. No. 11/730,719, filed Apr. 3, 2007, now U.S. Pat. No. 7,817,454,the disclosures of which are herewith incorporated by reference in theirentirety.

FIELD OF THE INVENTION

Embodiments of the invention relate to semiconductor devices, and inparticular, to variable resistance memory arrays and methods of formingand using the same.

BACKGROUND OF THE INVENTION

Non-volatile memories are useful storage devices due to their ability tomaintain data absent a power supply. Materials have been investigatedfor use in non-volatile memory cells. One class of programmableresistance materials are phase change materials, such as chalcogenidealloys, which are capable of stably transitioning between amorphous andcrystalline phases. Each phase exhibits a particular resistance stateand the resistance states distinguish the logic values of a memoryelement formed with such materials. Specifically, an amorphous stateexhibits a relatively high resistance, and a crystalline state exhibitsa relatively low resistance.

A conventional phase change memory element 1, illustrated in FIGS. 1Aand 1B, often has a layer of phase change material 8 between first andsecond electrodes 2, 4. The first electrode 2 is within a dielectricmaterial 6. The phase change material 8 is set to a particularresistance state according to the amount of current applied between thefirst and second electrodes 2, 4. To obtain an amorphous state (FIG.1B), a relatively high write current pulse (a reset pulse) is appliedthrough the phase change memory element 1 to melt at least a portion 9of the phase change material 8 covering the first electrode 2 for afirst period of time. The current is removed and the phase changematerial 8 cools rapidly to a temperature below the crystallizationtemperature, which results in the portion 9 of the phase change material8 covering the first electrode 2 having the amorphous state. To obtain acrystalline state (FIG. 1A), a lower current write pulse (a set pulse)is applied to the phase change memory element 1 for a second period oftime (typically longer in duration than the first period of time andcrystallization time of amorphous phase change material) to heat theamorphous portion 9 of the phase change material 8 to a temperaturebelow its melting point, but above its crystallization temperature. Thiscauses the amorphous portion 9 of the phase change material 8 tore-crystallize to the crystalline state that is maintained once thecurrent is removed and the phase change memory element 1 is cooled. Thephase change memory element 1 is read by applying a read voltage, whichdoes not change the phase state of the phase change material 8.

One drawback of conventional phase change memory elements is the largeprogramming current needed to achieve the phase change. This requirementleads to a large access transistor to achieve adequate current drive.Accordingly, it is desirable to have phase change memory elements withreduced programming requirements. It is also desirable to implementnovel transistors with a large current drive or provide an innovativecircuit layout that can provide more transistor current drive within thesame silicon area or both.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a cross-sectional view of a conventionalphase change memory element.

FIG. 2 illustrates a top view of a phase change memory array accordingto a first embodiment.

FIG. 3A illustrates an expanded top view of the phase change memoryarray of FIG. 2.

FIG. 3B illustrates a cross-section taken along line 3B-3B of the phasechange memory array of FIG. 3A.

FIG. 4A illustrates an expanded top view of the phase change memoryarray of FIG. 2 at an initial stage of a first method of fabrication.

FIG. 4B illustrates a cross-section taken along line 4B-4B of the phasechange memory array of FIG. 4A.

FIG. 5A illustrates a top view of the phase change memory array of FIG.2 at a stage of fabrication subsequent to FIG. 4A.

FIG. 5B illustrates a cross-section taken along line 5B-5B of the phasechange memory array of FIG. 5A.

FIG. 6A illustrates an expanded top view of the phase change memoryarray of FIG. 2 at an initial stage of a second method of fabrication.

FIG. 6B illustrates a cross-section taken along line 6B-6B of the phasechange memory array of FIG. 6A.

FIG. 7A illustrates a top view of the phase change memory array of FIG.2 at a stage of fabrication subsequent to FIG. 6A.

FIG. 7B illustrates a cross-section taken along line 7B-7B of the phasechange memory array of FIG. 7A.

FIG. 8A illustrates a top view of the phase change memory array of FIG.2 at a stage of fabrication subsequent to FIG. 7A.

FIG. 8B illustrates a cross-section taken along line 8B-8B of the phasechange memory array of FIG. 8A.

FIG. 9A illustrates a top view of the phase change memory array of FIG.2 at a stage of fabrication subsequent to FIG. 8A.

FIG. 9B illustrates a cross-section taken along line 9B-9B of the phasechange memory array of FIG. 9A.

FIG. 10A illustrates a top view of the phase change memory array of FIG.2 at a stage of fabrication subsequent to FIG. 9A.

FIG. 10B illustrates a cross-section taken along line 10B-10B of thephase change memory array of FIG. 10A.

FIG. 11A illustrates an expanded top view of the phase change memoryarray of FIG. 2 at an initial stage of a third method of fabrication.

FIG. 11B illustrates a cross-section taken along line 11B-11B of thephase change memory array of FIG. 11A.

FIG. 12A illustrates a top view of the phase change memory array of FIG.2 at a stage of fabrication subsequent to FIG. 11A.

FIG. 12B illustrates a cross-section taken along line 12B-12B of thephase change memory array of FIG. 12A.

FIG. 13A illustrates an expanded top view of the phase change memoryarray of FIG. 2 at an initial stage of a fourth method of fabrication.

FIG. 13B illustrates a cross-section taken along line 13B-13B of thephase change memory array of FIG. 13A.

FIG. 14 illustrates a top view of a phase change memory array accordingto a second embodiment.

FIG. 15A illustrates an expanded top view of the phase change memoryarray of FIG. 14.

FIG. 15B illustrates a cross-section taken along line 15B-15B of thephase change memory array of FIG. 15A.

FIG. 16 illustrates a top view of a phase change memory array accordingto a third embodiment.

FIG. 17A illustrates an expanded top view of the phase change memoryarray of FIG. 16 at an initial stage of fabrication.

FIG. 17B illustrates a cross-section taken along line 17B-17B of thephase change memory array of FIG. 17A.

FIG. 18A illustrates a top view of the phase change memory array of FIG.16 at a stage of fabrication subsequent to FIG. 17A.

FIG. 18B illustrates a cross-section taken along line 18B-18B of thephase change memory array of FIG. 18A.

FIG. 19A illustrates a top view of the phase change memory array of FIG.16 at a stage of fabrication subsequent to FIG. 18A.

FIG. 19B illustrates a cross-section taken along line 19B-19B of thephase change memory array of FIG. 19A.

FIG. 20A illustrates a top view of the phase change memory array of FIG.16 at a stage of fabrication subsequent to FIG. 19A.

FIG. 20B illustrates a cross-section taken along line 20B-20B of thephase change memory array of FIG. 20A.

FIG. 21A illustrates a top view of the phase change memory array of FIG.16 at a stage of fabrication subsequent to FIG. 20A.

FIG. 21B illustrates a cross-section taken along line 21B-21B of thephase change memory array of FIG. 21A.

FIG. 22 illustrates a top view of a phase change memory array accordingto a fourth embodiment.

FIG. 23 illustrates a top view of a phase change memory array accordingto a fifth embodiment.

FIG. 24 illustrates a top view of a phase change memory array accordingto a sixth embodiment.

FIG. 25A illustrates an expanded top view of the phase change memoryarray of FIG. 24 at an initial stage of fabrication.

FIG. 25B illustrates a cross-section taken along line 25B-25B of thephase change memory array of FIG. 25A.

FIG. 26A illustrates an expanded top view of the phase change memoryarray of FIG. 24 at a stage of fabrication subsequent to FIG. 25A.

FIG. 26B illustrates a cross-section taken along line 26B-26B of thephase change memory array of FIG. 26A.

FIG. 27A illustrates an expanded top view of the phase change memoryarray of FIG. 24 at a stage of fabrication subsequent to FIG. 26A.

FIG. 27B illustrates a cross-section taken along line 27A-27A of thephase change memory array of FIG. 27A.

FIG. 28 illustrates a top view of a phase change memory array accordingto a seventh embodiment.

FIG. 29 illustrates a top view of a phase change memory array accordingto an eighth embodiment.

FIG. 30A illustrates an expanded top view of the phase change memoryarray of FIG. 28 at an initial stage of fabrication.

FIG. 30B illustrates a cross-section taken along line 30B-30B of thephase change memory array of FIG. 30A.

FIG. 31 illustrates a cross-section of the phase change memory array ofFIG. 28 at a stage of fabrication subsequent to FIG. 30A.

FIG. 32 illustrates a cross-section of the phase change memory array ofFIG. 28 at a stage of fabrication subsequent to FIG. 31.

FIG. 33 illustrates a top view of a phase change memory array accordingto a ninth embodiment.

FIG. 34 illustrates a top view of a phase change memory array accordingto a tenth embodiment.

FIG. 35 is a block diagram of a processor system having a memory elementincorporating a phase change memory array constructed in accordance withan embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to variousembodiments of the invention. These embodiments are described withsufficient detail to enable those skilled in the art to practice them.It is to be understood that other embodiments may be employed, and thatvarious structural, logical and electrical changes may be made.

The term “substrate” used in the following description may include anysupporting structure including, but not limited to, a semiconductorsubstrate that has an exposed substrate surface. A semiconductorsubstrate should be understood to include silicon, silicon-on-insulator(SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors,epitaxial layers of silicon supported by a base semiconductorfoundation, and other semiconductor structures, including those made ofsemiconductors other than silicon. When reference is made to asemiconductor substrate or wafer in the following description, previousprocess steps may have been utilized to form regions or junctions in orover the base semiconductor or foundation. The substrate also need notbe semiconductor-based, but may be any support structure suitable forsupporting an integrated circuit, including, but not limited to, metals,alloys, glasses, polymers, ceramics, and any other supportive materialsas is known in the art.

Embodiments are now explained with reference to the figures, throughoutwhich like reference numbers indicate like features. FIG. 2 illustratesa first embodiment, in which word lines 20 run horizontally andvertically in a square lattice configuration. Each word line 20 formstransistor gates which have source/drain regions on both sides of thegate. Phase change memory elements 25 are positioned within the latticeof the word lines 20, alternating horizontally and vertically withbitline contacts 26. Bitlines 21 run diagonally between bitline contacts26. For ease of illustration, not all bitlines are shown.

To program a selected phase change memory element 25 a, two adjacentvertical word lines 20 a and two adjacent horizontal word lines 20 benclosing the selected phase change memory element 25 a are turned on. Atop electrode select line 22 a that is in contact with the selectedphase change memory element 25 a is also selected. For ease ofillustration, not all top electrode select lines 22 are shown. All ofthe bitlines 21 are grounded or biased at the same voltage. The fourtransistors associated with the word lines 20 a, 20 b enclosing thephase change memory element 25 a are turned on to supply programmingcurrent to the element 25 a. Current flows from the selected topelectrode select line 22 a through the transistors associated with theword lines surrounding the phase change memory element 25 a into thenearest bitline contacts 26 a.

Turning now to FIG. 3A, an expanded top view of a portion of the phasechange memory array of FIG. 2 is shown. The selected phase change memoryelement 25 a is enclosed by word lines 20 a, 20 b. FIG. 3B illustrates across-section taken along line 3B-3B of the phase change memory array ofFIG. 3A. Top electrode select lines 22 run above the phase change memoryelements 25, contacting their top electrodes. When the selected topelectrode select line 22 a is turned on, current is supplied by theselected top electrode select line 22 a and passes through the selectedphase change memory element 25 a. Since the bitlines 21 are grounded orbiased at the same voltage, the current through the selected phasechange memory element 25 a goes across all four transistors defined byfour segments of word lines 20 a, 20 b to adjacent bitline contacts 26a.

FIGS. 4A-5B illustrate a first method of forming the phase change memoryarray of FIG. 2. FIG. 4A is an expanded top view of the memory array atan initial stage of fabrication according to the first method. FIG. 4Bis a cross-section of FIG. 4A, taken across line 4B-4B. A first array ofvertically-aligned word lines 20 are formed on a silicon substrate 10using any known fabrication method. An ion implantation process may beperformed to dope regions in the silicon that are not protected by thevertically-aligned word lines 20 so that the desired silicon dopingprofile is preserved. No trench isolation regions are necessary.

A cleaning process may be performed to remove damaged oxide on thesilicon substrate 10 before forming a second array ofhorizontally-aligned word lines 20′, as shown in FIGS. 5A and 5B.Methods such as photolithography and dry etching may be used to form thehorizontally-aligned word lines 20′. The horizontally-aligned word lines20′ are perpendicular to the vertically-aligned word lines 20. Anoptional strip of nitride spacers may be formed on the word lines 20,before source/drain regions 23 are formed by one or more high-doseimplants. A silicide metal such as Co, Ni, or Ti is deposited forsilicidation (or salicidation if the gate stacks of the word lines arepolysilicon/TEOS gate stacks) of the source/drain regions 23.

Self-aligned metal contacts and bitline contacts 26 a are formed overthe source/drain regions 23, as shown in FIG. 3B. Material for bitlines21 are deposited and patterned. The phase change memory elements 25 areformed in layers in the shape of mesas or stripes, as shown in FIGS. 1Aand 1B, and the top electrode select lines 22 are formed with a contactto the top electrode 4 of the phase change memory element 25, which isin contact with the phase change memory material 8 having a portion 9 incontact with the bottom electrode 2. Depending upon the desiredorientation of the top electrode select lines 22, they may be providedin one or more layers, as long as no two adjacent phase change memoryelements 25 are contacted by the same top electrode select line 22.

In a second method of forming the phase change memory array of FIG. 2,word line gate materials 127 are deposited over a silicon substrate 110,as shown in FIGS. 6A and 6B. FIG. 6B is a cross-section taken acrossline 6B-6B in the expanded top view of FIG. 6A. The silicon substrate110 may be provided with ion implantation to define a desired dopantprofile. Photolithograpy and dry etch processes may be used to etch anarray of square patterns into the silicon substrate 110, and filled withhigh-density plasma (HDP) oxide to form shallow trench isolation (STI)regions 128.

As shown in FIG. 7A, a resist pattern 137 is provided over the substrate110, such that strips of resist material intersect perpendicularly overthe STI regions 128. FIG. 7B illustrates a cross-section taken acrossline 7B-7B in the expanded top view of 7A.

A photolithography and dry etch process is performed to producevertically- and horizontally-aligned gate stacks of word lines 120, 120′that intersect over the STI regions 128, as shown in the expanded topview of FIG. 8A. The photolithography and dry etch process is used toetch isolated gate stacks of word lines 120, 120′, stopping above thesilicon substrate 110, as shown in the cross-section illustrated in FIG.8B, taken across line 8B-8B of FIG. 8A. Nitride spacers 120″ are formedto complete the formation of the transistors and source/drain regions123 are formed. A silicide metal (such as Co, Ni or Ti) is deposited forsource/drain silicidation (or salicidation for polysilicon/TEOS gatestacks).

Because the gate stacks of word lines 120, 120′ are isolated from eachother, they must be electrically connected in order to form continuousword lines. FIG. 9A illustrates an expanded top view of this connectionand FIG. 9B is a cross-section taken along line 9B-9B of FIG. 9A. Asshown in FIG. 9B, contacts 130 are formed over the vertically-alignedgate stacks of word lines 120 to electrically connect thevertically-aligned gate stacks of word lines 120 with vertically-alignedstraps 129. Contacts 130′ are formed over the horizontally-aligned gatestacks of word lines 120′ to electrically connect the horizontallyaligned gate stacks of word lines 120′ with horizontally-aligned straps129′. Both vertically- and horizontally-aligned straps 129, 129′ aretypically conductive metal lines having a nitride encapsulating layerprovided over them to electrically isolate the straps 129, 129′.

Depending upon the desired orientation of the top electrode select lines122, they may be provided in one or more layers, as long as no twoadjacent phase change memory elements 125 are contacted by the same topelectrode select line 122, as shown in FIG. 10B, which is across-section of expanded top view 10A taken along line 10B-10B.

In a third method of forming the phase change memory array of FIG. 2,gate materials 227 are deposited over a silicon substrate 210, as shownin FIGS. 11A and 11B. FIG. 11B is a cross-section taken across line11B-11B in the expanded top view of FIG. 11A. The silicon substrate 210may be provided with ion implantation to define a desired dopantprofile. A resist 227 is patterned as shown in FIG. 11A. The pattern ofthe resist 227 defines the location of the isolated gate stacks, as willbe described below.

A photolithography and dry etch process is performed to producevertically- and horizontally-aligned word lines 220, 220′, as shown inthe expanded top view of FIG. 12A. The photolithography and dry etchprocess is used to etch isolated gate stacks, stopping above the siliconsubstrate 210, as shown in the cross-section illustrated in FIG. 12B,taken across line 12B-12B of FIG. 12A. Nitride spacers 220″ are formedto complete the formation of the transistors and source/drain regions223 are formed. The remainder of the steps are performed in accordancewith the second method described above with respect to FIGS. 9A and 9B.

In a fourth method of forming the phase change memory array of FIG. 2, afirst array of parallel word lines 320 are formed on a substrate 310using a recessed transistor process, as shown in FIG. 13B, which is across-section taken across 13B-13B in expanded top view 13A. Because thebottom layer 321 of the recessed word lines 320 are formed withintrenches in the substrate 310, recessed word lines 320 have a lowertopography than the word lines in the arrays described above. By formingrecessed word lines 320, the second array of parallel word lines thatwill be formed perpendicular to the first array 320 may also have areduced topography. The remainder of the steps are performed inaccordance with the first method described above with respect to FIGS.1A, 1B, 3B, 5A and 5B.

A phase change memory array with word lines configured in a latticeconfiguration having enclosing transistors around the phase changememory elements can provide to each phase change memory element acurrent that is more than four times greater than a conventional planartransistor. At the same time, this array optimizes the silicon area bytaking advantage of the symmetry of the array to minimize the unit cellarea by sharing transistor source/drain regions with adjacenttransistors in a two-dimensional configuration. In the embodiment ofFIG. 2, the unit cell area is 8f² with more than four times thetransistor current drive than can be obtained from a one-transistorcurrent drive for a conventional 8f² unit cell layout. The circuitbiasing scheme is similar to the conventional planar transistor circuitswith perpendicular word lines and top electrode select lines. However,the fabrication process is simpler since no trench isolation regions areneeded for element isolation.

FIG. 14 illustrates a second embodiment in which, similar to theembodiment of FIG. 2, the word lines 20 run horizontally and verticallyin a square lattice configuration. The phase change memory elements 25are positioned within the lattice of the word lines 20, alternatinghorizontally and vertically with bitline contacts 26. The bitlines 21run diagonally between bitline contacts 26. For ease of illustration,not all bitlines are shown.

The top electrode select lines 322 have a “wavy” configuration such thatevery other diagonally adjacent phase change memory elements 25 are incontact with the same top electrode select line 322, but no two adjacentphase change memory elements 25 are in contact with the same topelectrode select line 322. For ease of illustration, not all topelectrode select lines are shown.

This configuration of top electrode select lines 322 has a benefit overthe configuration of FIG. 2, since fewer top electrode select lines 322are necessary and may be relatively easier to pattern.

Otherwise, the methods for forming the second embodiment illustrated inFIG. 14 are the same as the methods for forming the first embodimentillustrated in FIG. 2. As shown in the expanded top view of FIG. 15A andcross-section taken along line 15B-15B in FIG. 15B, the word lines 20,phase change memory elements 25, bitline contacts 26 and bitline 21 havethe same configuration as the embodiment in FIG. 2. Only the topelectrode select lines 322 have a different configuration, being curvedaround every other phase change memory element and making contact withevery other phase change memory element on a diagonal line.

FIG. 16 illustrates a third embodiment in which the word lines 420 a,420 b, 420 c run at 60 degree angles with respect to each other in ahexagonal lattice configuration. The phase change memory elements 425are positioned within a lattice formed of a first array of horizontalword lines 420 a, a second array of word lines 420 b rotated at a +60degree angle from the first array of word lines 420 a, and a third arrayof word lines 420 c rotated at a −60 degree angle from horizontal wordlines 420 a. The bitline contacts 426 are also positioned within thelattice formed of word lines 420 a, 420 b, 420 c, alternating with thephase change memory elements 425, so that no two adjacent enclosuresformed by word lines 420 a, 420 b, 420 c have phase change memoryelements 425 in them and no two adjacent enclosures formed by word lines420 a, 420 b, 420 c have bitline contacts 426 in them. The bitlinecontacts 426 may be individually addressed, or can be grounded or biasedat the same voltage. For ease of illustration, not all bitlines areshown.

To program a selected phase change memory element 425 a, the three wordlines 420 a′, 420 b′, 420 c′ enclosing the selected phase change memoryelement 425 a are turned on. A top electrode select line 422 a that isin contact with the selected phase change memory element 425 a is alsoselected. The top electrode select lines 422, although shown here with422 a in a straight line, may have any configuration since no two phasechange memory elements 425 are adjacent to each other. For ease ofillustration, not all top electrode select lines are shown. All of thebitline contacts 426 are grounded or biased at the same voltage. Thethree transistors enclosing the phase change memory element 425 a areturned on to supply programming current to the element 425 a. Currentflows from the selected top electrode select line 422 a through thephase change memory element 425 a into the three nearest bitlinecontacts 426 a.

The embodiment of FIG. 16 having word lines configured in a hexagonallattice configuration with three enclosing transistors around the phasechange memory elements can provide to each phase change memory element acurrent that is more than three times greater than a conventional planartransistor. At the same time, this array optimizes the silicon area bytaking advantage of the symmetry of the array to minimize the unit cellarea by sharing transistor source/drain regions with adjacenttransistors. In the embodiment of FIG. 16, the unit cell area is 2√3f².

Turning now to FIGS. 17A-21B, which illustrate the process by which theembodiment of FIG. 16 is formed, gate materials 427 are deposited over asilicon substrate 410, as shown in FIGS. 17A and 17B. FIG. 17Aillustrates an expanded top view of an initial stage of fabrication andFIG. 17B is a cross-section taken across line 17B-17B of FIG. 17A. Thesilicon substrate 410 may be provided with ion implantation to define adesired dopant profile. Photolithograpy and dry etch processes may beused to etch a hexagonal array pattern into the silicon substrate 410,and filled with high-density plasma (HDP) oxide to form shallow trenchisolation (STI) regions 428.

As shown in FIG. 18A, a resist pattern 437 is provided over thesubstrate 410, such that intersections are provided over the STI regions428. FIG. 18B illustrates a cross-section taken across line 18B-18B inthe expanded top view of 18A.

A photolithography and dry etch process is performed to produce gatestacks of word lines 420 a, 420 b, 420 c that intersect over the STIregions 128, as shown in the expanded top view of FIG. 19A. Thephotolithography and dry etch process is used to etch isolated gatestacks of word lines 420 a, 420 b, 420 c, stopping above the siliconsubstrate 410, as shown in the cross-section illustrated in FIG. 19B,taken across line 19B-19B of FIG. 19A. Nitride spacers are formed tocomplete the formation of the transistors and source/drain regions 423are formed. A silicide metal (such as Co, Ni or Ti) is deposited forsource/drain silicidation (or salicidation for polysilicon/TEOS gatestacks).

Because the gate stacks of word lines 420 a, 420 b, 420 c are isolated,they must be electrically connected in order to form word lines. FIG.20A illustrates an expanded top view of this connection and FIG. 20B isa cross-section taken along line 20B-20B of FIG. 20A. As shown in FIG.20B, contacts 430 a are formed to electrically connect the gate stacksof the first array of word lines 420 a to a first array ofhorizontally-aligned straps 429 a. Contacts 430 b are formed toelectrically connect the gate stacks of the second array of word lines420 b with a second array of straps 429 b, which are positioned alongthe second array of word lines 420 b. Contacts 430 c are formed toelectrically connect the gate stacks of the third array of word lines420 c to a third array of straps 429 c, which are positioned along thethird array of word lines 420 c. All three arrays of straps 429 a, 429b, 429 c are typically conductive metal lines having a nitrideencapsulating layer 431 a, 431 b, 431 c provided over them toelectrically isolate the straps 429 a, 429 b, 429 c.

A plurality of top electrode select lines 422 are provided in contactwith the top electrodes of the phase change memory elements 425,however, no two adjacent phase change memory elements 425 are connectedto the same top electrode select lines 422, as shown in FIGS. 21A and21B. It should be understood that, for simplicity of illustration, thetransistors and straps connecting them are represented as word lines 420a, 420 b, 420 c.

The embodiment of FIG. 16 having word lines configured in a hexagonallattice configuration may also be fabricated with one word line arrayusing a recessed transistors, while the other two word line arrays areconventional transistors, or with all three word line arrays havingconventional transistors, as described above. Another method of formingthe embodiment of FIG. 16 may be the third method described above withrespect to FIGS. 9A, 9B and 11A-12B, which employs photo-patterning anddry etch techniques to form the enclosing gate stacks.

FIG. 22 illustrates a fourth embodiment in which the word lines 520 havea “ladder-shaped” configuration, consisting of two parallel segments520′ and shorter segments 520″ connecting the two parallel segments520′. The two parallel segments 520′ run on either side of a column ofalternating phase change memory elements 525 and bitline contacts 526,while the shorter segments 520″ are positioned between the phase changememory elements 525 and the bitline contacts 526. The bitline contacts526 may all be grounded or biased at the same voltage. For ease ofillustration, not all bitlines are shown.

To program a selected phase change memory element 525 a, the word line520 a enclosing the selected phase change memory element 525 a is turnedon. A top electrode select line 522 a that is in contact with theselected phase change memory element 525 a is also selected. For ease ofillustration, not all top electrode select lines are shown. The fourtransistors of selected word line 520 a enclosing the phase changememory element 525 a are turned on to supply programming current to theelement 525 a. Current flows from the selected top electrode select line522 a through the phase change memory element 525 a to the commonsource/drain region of the transistors of the word lines 522 a andacross the transistors to the common source/drain regions to the nearestbitline contacts 526 a.

The embodiment of FIG. 22 having word lines configured in a “ladder”lattice configuration with four enclosing transistors around the phasechange memory elements can provide to each phase change memory element acurrent that is at least four times greater than a conventional planartransistor. At the same time, this array optimizes the silicon area bytaking advantage of the symmetry of the array to minimize the unit cellarea by sharing transistor source/drain regions with adjacenttransistors. In the embodiment of FIG. 22, the unit cell area is lessthan 14f².

FIG. 23 illustrates a fifth embodiment in which the word lines 620 havea “rounded ladder-shaped” configuration, consisting of rings 620′enclosing the phase change memory elements 625 that are connected bysegments 620″ that enclose bitline contacts 626. The bitline contacts626 and phase change memory elements 625 are alternately positioned incolumns and rows, with at least a ring 620′ and/or segment 620″ betweenthem. Because the word lines 620 are curved, the transistor effectivewidth of the word lines 620 is increased when compared to a straightword line in the same configuration. The unit cell area is less than14f².

FIG. 24 illustrates a sixth embodiment in which the word lines 720 havea ladder-shaped configuration, consisting of two parallel segments 720′and rung segments 720″ connecting the two parallel segments 720′. Thetwo parallel segments 720′ run on either side of a column of phasechange memory elements 725, with the rung segments 720″ being positionedbetween the phase change memory elements 725. Bitline contacts 726 arepositioned within the rows of phase change memory elements 725,alternating with the phase change memory elements 725, and placedbetween the word lines 720. The bitline contacts 726 may all be groundedor biased at the same voltage. For ease of illustration, not allbitlines are shown.

To program a selected phase change memory element 725 a, the word line720 a enclosing the selected phase change memory element 725 a is turnedon. A top electrode select line 722 a that is in contact with theselected phase change memory element 725 a is also selected. For ease ofillustration, not all top electrode select lines are shown. The fourtransistors enclosing the phase change memory element 725 a are turnedon to supply programming current to the element 725 a. Current flowsfrom the selected top electrode select line 722 a through the phasechange memory element 725 a across the transistors of the selected wordlines 720 a to the adjacent bitline contacts 726 a. Current also flowsthrough the transistors 720″ to the common source/drain region of thetransistors 720″ and a neighboring transistor 720′ to the adjacentbitline contacts 726 b.

The embodiment of FIG. 24 having word lines configured in a ladderlattice configuration with four enclosing transistors around the phasechange memory elements can provide to each phase change memory element acurrent that is at least three times greater than a conventional planartransistor. At the same time, this array optimizes the silicon area bytaking advantage of the symmetry of the array to minimize the unit cellarea by sharing transistor source/drain regions with adjacenttransistors. In the embodiment of FIG. 24, the unit cell area isapproximately 8f².

FIGS. 25A-27B illustrate a first method of forming the phase changememory array of FIG. 24. FIG. 25A is an expanded top view of the memoryarray at an initial stage of fabrication. FIG. 25B is a cross-section ofFIG. 25A, taken across line 25B-25B. An ion implantation process may beperformed to define a desired dopant profile in the silicon substrate710. An array ladder-like word lines 720 are patterned on a siliconsubstrate 710 by photolithography and dry etch processes.

Turning now to FIGS. 26A and 26B, nitride spacers may be formed on theword lines 720 before source/drain regions 723 are formed by one or morehigh-dose implants. A silicide metal such as Co, Ni, or Ti is depositedfor silicidation (or salicidation if the gate stacks of the word linesare polysilicon/TEOS gate stacks) of the source/drain regions 723.

Self-aligned metal contacts and bitline contacts 726 are formed over thesource/drain regions 723, as shown in FIGS. 27A and 27B. Material forbitlines 721 are deposited and patterned. The phase change memoryelements 725 are formed in layers, as shown in FIGS. 1A and 1B, and thetop electrode select lines 722 a are formed with a contact to the topelectrode 4 of the phase change memory elements 725.

FIG. 28 illustrates a seventh embodiment in which the word lines 820have a “diamond” lattice configuration, enclosing the phase changememory elements 825 with four transistors in a diamond-shapedconfiguration. Bitline contacts 826 are positioned between columns ofdiamond-shaped word lines 820. More or fewer bitline contacts 826 may beprovided than are shown. The bitline contacts 826 may all be grounded orbiased at the same voltage.

To program a selected phase change memory element 825 a, the word line820 a enclosing the selected phase change memory element 825 a is turnedon. A top electrode select line 822 a that is in contact with theselected phase change memory element 825 a is also selected. For ease ofillustration, not all top electrode select lines are shown. The fourtransistors enclosing the phase change memory element 825 a are turnedon to supply programming current to the element 825 a. Current flowsfrom the selected top electrode select line 822 a through the phasechange memory element 825 a into the common source/drain regionssurrounding the enclosing transistors to the adjacent bitline contacts826 a.

The embodiment of FIG. 28 having word lines configured in a diamondlattice configuration with four enclosing transistors around the phasechange memory elements can provide to each phase change memory element acurrent that is at least four times greater than a conventional planartransistor. At the same time, this array optimizes the silicon area bytaking advantage of the symmetry of the array to minimize the unit cellarea by sharing transistor source/drain regions with adjacenttransistors. In the embodiment of FIG. 28, the unit cell area is lessthan 9.5f².

FIG. 29 illustrates an eighth embodiment, which is a variation on FIG.28 having a phase change memory array with word lines 920 a in a diamondlattice configuration. However, top electrode select line 922 a is wavy,and runs in a perpendicular line across the plurality of word lines 920and phase change memory elements 925, 925 a.

FIGS. 30A-32 illustrate a method of forming the phase change memoryarray of FIG. 28. FIG. 30A is an expanded top view of the memory arrayat an initial stage of fabrication. FIG. 30B is a cross-section of FIG.30A, taken across line 30B-30B. An ion implantation process may beperformed to define a desired dopant profile in the silicon substrate810. An array of diamond-like word lines 820 are patterned on a siliconsubstrate 810 by photolithography and dry etch processes.

Turning now to FIG. 31, nitride spacers may be formed on the word lines820 before source/drain regions 823 are formed by one or more high-doseimplants. A silicide metal such as Co, Ni, or Ti is deposited forsilicidation (or salicidation if the gate stacks of the word lines arepolysilicon/TEOS gate stacks) of the source/drain regions 823.

Self-aligned metal contacts and bitline contacts 826 are formed over thesource/drain regions 823, as shown in FIG. 32. Material for bitlines 821are deposited and patterned. The phase change memory element 825 isformed in layers, as shown in FIGS. 1A and 1B, and the top electrodeselect line 822 is formed with a contact to the top electrode 4 of thephase change memory elements 825. A similar method may be employed toform the phase change memory array of FIG. 29.

FIG. 33 illustrates a ninth embodiment in which the word lines 1020 havea “triangular” lattice configuration, enclosing the phase change memoryelements 1025 with three transistors in a triangle configuration.Bitline contacts 1026 may be positioned near the apex of thetriangle-shaped word lines 1020 or other locations outside of the threeenclosing transistors. The bitline contacts 1026 may all be grounded orbiased at the same voltage.

To program a selected phase change memory element 1025 a, the word line1020 a enclosing the selected phase change memory element 1025 a isturned on. A top electrode select line 1022 a that is in contact withthe selected phase change memory element 1025 a is also selected. Forease of illustration, not all top electrode select lines are shown. Thethree transistors enclosing the phase change memory element 1025 a areturned on to supply programming current to the element 1025 a. Currentflows from the selected top electrode select line 1022 a through thephase change memory element 1025 a across the transistors of theenclosing word lines 1020 and into the common source/drain regions tothe adjacent bitline contacts 1026 a.

The embodiment of FIG. 33 having word lines 1020 configured in atriangular lattice configuration with three enclosing transistors aroundthe phase change memory elements can provide to each phase change memoryelement a current that is about five times greater than a conventionalplanar transistor. At the same time, this array optimizes the siliconarea by taking advantage of the symmetry of the array to minimize theunit cell area by sharing transistor source/drain regions with adjacenttransistors. In the embodiment of FIG. 33, the unit cell area is lessthan 16f².

FIG. 34 illustrates a tenth embodiment which is a variation on FIG. 33having a phase change memory array with word lines 1120 a in atriangular lattice configuration. However, top electrode select line1122 a is straight, and runs in an angle across the plurality of wordlines 1120 and phase change memory elements 1125, 1125 a.

FIG. 35 illustrates a simplified processor system 100 which includes amemory circuit 106 having a phase change memory array constructed inaccordance with the invention.

The FIG. 35 processor system 100, which can be any system including oneor more processors, for example, a computer, PDA, phone or other controlsystem, generally comprises a central processing unit (CPU) 102, such asa microprocessor, a digital signal processor, or other programmabledigital logic devices, which communicates with an input/output (I/O)device 105 over a bus 101. The memory circuit 106 communicates with theCPU 102 over bus 101 typically through a memory controller. The memorycircuit 106 includes one or more of the phase change memory arraysdepicted in FIGS. 2, 14, 16, 22-24, 28, 29, 33 and/or 34.

In the case of a computer system, the processor system 100 may includeperipheral devices such as a compact disc (CD) ROM drive 103 and harddrive 104, which also communicate with CPU 102 over the bus 101. Ifdesired, the memory circuit 106 may be combined with the processor, forexample, CPU 102, in a single integrated circuit.

While various embodiments have been described herein as relating to aphase change memory arrays, it should be appreciated that the latticearrays and transistor arrangements described herein may be used withother variable resistance memory technologies and other technologiesthat require high programming current. Examples of such memorytechnologies include MRAM, RRAM, STT (Spin-Torque-Transfer), and thelike.

The above description and drawings are only to be consideredillustrative of specific embodiments, which achieve the features andadvantages described herein. Modification and substitutions to specificprocess conditions and structures can be made. Accordingly, theembodiments of the invention are not to be considered as being limitedby the foregoing description and drawings, but is only limited by thescope of the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of forming a memory array comprising:forming a plurality of word lines over a substrate to form groups of atleast three transistors, each group sharing one of a plurality of commonsource/drain regions; forming a phase change memory element electricallycoupled to a common source/drain region shared by a first group oftransistors; forming a bitline contact electrically coupled to a commonsource/drain region shared by a second group of transistors, wherein thecommon source/drain region coupled to the bitline contact iselectrically coupled to the common/source drain region coupled to thephase change memory element through a transistor common to the first andsecond groups of transistors; and forming a top electrode select lineelectrically coupled to the phase change memory element, wherein formingthe plurality of word lines further comprises: forming a first pluralityof isolated gate stacks along a first direction over the substrate, asecond plurality of isolated gate stacks along a second direction overthe substrate, and a third plurality of isolated gate stacks along athird direction over the substrate, wherein the first, second, and thirddirections comprise a hexagonal array pattern; and forming a firstelectrical connection between the first plurality of gate stacks to formthe first plurality of word lines, a second electrical connectionbetween the second plurality of gate stacks to form the second pluralityof word lines, and a third electrical connection between the thirdplurality of gate stacks to form the third plurality of word lines.
 2. Amethod of forming a memory array comprising: forming a plurality of wordlines to form groups of at least three transistors, each group sharingone of a plurality of common source/drain regions; forming a phasechange memory element electrically coupled to a common source/drainregion shared by a first group of transistors; forming a bitline contactelectrically coupled to a source/drain region of one of the transistorsof the first group, which is electrically coupled to the commonsource/drain region; and forming a top electrode select line in contactwith the phase change memory element, wherein forming the plurality ofword lines comprises forming the plurality of word lines into aplurality of substantially triangle word line shapes.
 3. A method offorming a memory array comprising: forming a first plurality of wordlines over a substrate extending in a first direction; forming a secondplurality of word lines electrically isolated from the first pluralityof word lines over the substrate extending in a second direction;forming a plurality of source/drain regions on opposite sides of each ofthe first and second plurality of word lines such that the first andsecond word lines define a group of transistors which share commonsource/drain regions; forming a phase change memory element electricallycoupled to a first common source/drain region of one of the transistorsof a first group; forming a bitline contact electrically coupled to asecond common source/drain region of the one of the transistors of thefirst group; forming a top electrode select line electrically coupled tothe phase change memory element; and forming a bitline electricallycoupled to the bitline contact; wherein, the first and second commonsource/drain regions are electrically coupled through the one transistorof the first group.
 4. The method of claim 3, wherein the first andsecond plurality of word lines are formed as isolated gate stacks whichextend in first and second directions; the method further comprising;electrically coupling the isolated gate stacks arranged in the firstdirection to form the first plurality of word lines; and electricallycoupling the isolated gate stacks arranged in the second direction toform the second plurality of word lines.
 5. The method of claim 3,further comprising; etching an array of square patterns into thesubstrate; and forming a shallow trench isolation regions in thesubstrate by filling the etched square patterns with oxide; wherein thefirst and second plurality of word lines are formed over the substrateto intersect over the shallow trench isolation regions.
 6. The method ofclaim 3, wherein forming the first plurality of word lines comprisesforming a first array of parallel word lines on the substrate by arecessed transistor process.
 7. The method of claim 3, wherein the firstplurality of word lines are perpendicular to the second plurality ofword lines.
 8. The method of claim 3, wherein the top electrode selectline is diagonal to either the first or second plurality of word lines.9. The method of claim 3, wherein the top electrode select line is wavy.10. The method of claim 3, wherein the bitline is diagonal to either thefirst or second plurality of word lines.
 11. The method of claim 3,further comprising: forming a nitride spacers on either the first orsecond plurality of word lines before forming the common source/drainregions.
 12. A method of claim 3, further comprising: forming a thirdplurality of word lines electrically isolated from the first and secondplurality of word lines over the substrate extending in a thirddirection; and forming a plurality of source/drain regions on thesubstrate on the opposite sides of the third plurality of word linessuch that the first, second and third word lines define groups oftransistors which share common source/drain regions.
 13. The method ofclaim 12, wherein forming the second plurality of word lines comprisesforming the second plurality of word lines at a substantially 60 degreeangle to the first plurality of word lines.
 14. The method of claim 12,wherein forming the third plurality of word lines comprises forming thethird plurality of word lines at a substantially negative 60 degreeangle to the first plurality of word lines.
 15. A method of forming amemory array comprising: forming a plurality of word lines over asubstrate; forming a plurality of source/drain regions on the substrateon opposite sides of the plurality of word lines, wherein the word linesand the source/drain regions formed on the opposite sides thereof form aplurality of transistors, and the source/drain regions are shared by theplurality of transistors; forming a phase change memory elementelectrically coupled to a first source/drain region of one of theplurality of transistors; forming a bitline contact electrically coupledto a second source/drain region of the one of the plurality oftransistors; forming a top electrode select line electrically coupled tothe phase change memory element; and forming a bitline electricallycoupled to the bitline contact; wherein the word lines are formed overthe substrate such that a group of transistors surround the firstsource/drain region; wherein the bitline is parallel to and does notoverlap with the word lines; wherein the word lines form ladder shapepatterns over the substrate; wherein the first source/drain region isformed within an enclosed area of a ladder shape pattern; and whereinthe ladder shape is a round ladder shape.
 16. A method of forming amemory array comprising: forming a plurality of word lines over asubstrate; forming a plurality of source/drain regions on the substrateon opposite sides of the plurality of word lines, wherein the word linesand the source/drain regions formed on the opposite sides thereof form aplurality of transistors, and the source/drain regions are shared by theplurality of transistors; forming a phase change memory elementelectrically coupled to a first source/drain region of one of theplurality of transistors; forming a bitline contact electrically coupledto a second source/drain region of the one of the plurality oftransistors; forming a top electrode select line electrically coupled tothe phase change memory element; and forming a bitline electricallycoupled to the bitline contact; wherein the word lines are formed overthe substrate such that a group of transistors surround the firstsource/drain region; wherein the bitline is parallel to and does notoverlap with the word lines; wherein the word lines form triangularshape patterns over the substrate; and wherein the first source/drainregion is formed within an enclosed area of a triangular shape pattern.17. A method of forming a memory array comprising: forming a pluralityof word lines over a substrate; forming a plurality of source/drainregions on the substrate on opposite sides of the plurality of wordlines, wherein the word lines and the source/drain regions formed on theopposite sides thereof form a plurality of transistors, and thesource/drain regions are shared by the plurality of transistors; forminga phase change memory element electrically coupled to a firstsource/drain region of one of the plurality of transistors; forming abitline contact electrically coupled to a second source/drain region ofthe one of the plurality of transistors; forming a top electrode selectline electrically coupled to the phase change memory element; andforming a bitline electrically coupled to the bitline contact; whereinthe word lines are formed over the substrate such that a group oftransistors surround the first source/drain region; wherein the bitlineis parallel to and does not overlap with the word lines; and wherein thetop electrode select line is wavy.